The market demand for non-volatile semiconductor memory devices with higher and higher storage capacity is known. The manufacturers are therefore constantly coping with the task of finding solutions suitable for achieving the desired storage capacity at the same time keeping the production costs reasonable.
A promising technique provides for storing in each single non-volatile memory cell more than just one bit of information. This technique is also known as multiple-level programming and sensing. While in conventional, bilevel non-volatile memory devices each memory cell can take only two distinct programming states, i.e., written and erased, in multiple-level devices each memory cell can take more than two programming states, for example four, corresponding to two bits of information.
High capacity multiple-level Flash EEPROM memories have been presented by some manufacturers. However, such devices exhibit significant reliability problems. Multiple-level programming and sensing requires that the different programming levels of the memory cells be more precisely set than in the bilevel programming and sensing case, with a degree of precision increasing with the number of programming levels to be programmed and detected. Flash EEPROM devices are characterized by the feature of electrical erasing, and as a consequence the features of the memory cells vary during the life of the device.